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By Reviewed By Andreas Zabczyk Mar 05, 2009 Updated Dec 09, 2019

Agatized Fossil Coral

Fossil Coral from Indonesia
Fossil Coral from Indonesia

Fossil coral is a natural stone that is formed when ancient coral is gradually replaced with agate. The proper name for this material is agatized coral or agatized fossil coral. The fossilized coral typically appears as small flower-like patterns in the stone.

Agatized fossil coral can exhibit a wide range of natural colors, ranging from white and pink to brown, gray, black, yellow and red. Coral gemstones may vary widely depending on weathering, oxidation and the original mineral content.

Corals are among the most colorful animals in the sea. Most corals live in warm, shallow, tropical seas and feed on plankton. The oldest fossil corals are from the Ordovican period, which began over 450 million years ago. Corals are marine animals with a sac-like body, a mouth, tentacles and a skeleton. It is the skeleton that survives in the fossil record. Corals may be solitary or form large colonies.

Natural Fossil Coral
Natural Fossil Coral

In some cases corals were buried in sediment as the land surface was affected by crystal plate movements or as the oceans rose and fell. Temperature and pressure from compaction during burial resulted in those deposits becoming rock and part of the present day geological record.

Agate, also known as chalcedony, is a type of cryptocrystalline quartz or silicon dioxide. Under unique geological conditions, prehistoric corals and mollusks can fossilize by being replaced with agate from silica-rich ground water percolating through limestone.

Of the varieties of fossilized corals found throughout the world, exquisitely detailed specimens from the mountains of Indonesia are among the most unique. In Indonesia, entire coral heads are often completely preserved and appear just as they did 20 million years ago, though their density is much changed by replacement with silica, iron, manganese and other minerals. There are fern corals, brain corals, hex corals, honeycomb corals and many more.

Florida and Georgia in the USA are also important sources of fossil coral. In fact Florida has declared fossil coral to be the Florida State Rock.

Fossil coral pieces are sought by gemstone and fossil collectors around the world. Additionally, fossil coral gemstones are used in beads and other jewelry. Since agate is fairly hard (7 on the Mohs scale) and very durable, fossil coral is suitable for all kinds of jewelry.

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